1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer, a copying machine, a facsimile and the like for executing an image formation by adopting an image forming process including a step of uniformly charging an image bearing member with a predetermined polarity and potential to the image bearing member such as an electrophotographic photosensitive member and an electrostatic recording dielectrics.
More specifically, the present invention relates to the image forming apparatus of xe2x80x9ccleaning simultaneous with developingxe2x80x9d and xe2x80x9ccleanerlessxe2x80x9d by a contact charging system.
2. Description of the Related Art
To describe the image forming apparatus utilizing the electrophotographic system as an example, there is available an image forming apparatus referred to as xe2x80x9ccleaning simultaneous with developingxe2x80x9d and xe2x80x9ccleanerlessxe2x80x9d wherein, with the object of miniaturizing the whole apparatus, keeping up with the trend of ecology-consciousness without generating a waste toner, allowing a photosensitive drum which is the image bearing member to have a long life and reducing the consumption amount of the toner which is a developer for one page portion, developing means is used as cleaning means for cleaning the toner (hereinafter referred to as the transfer residual toner) which remains on the surface of the photosensitive drum after a toner image has been transferred to a material to be transferred, thereby abolishing the installation of cleaning means as an exclusive device.
(a) In the above described image forming apparatus of xe2x80x9ccleaning simultaneous with developingxe2x80x9d and xe2x80x9ccleanerlessxe2x80x9d, the charging means has been disclosed, for example, in Japanese Patent Application Laid-Open No. 04-20986. This charging means allows a rotary brush, which is an electrically conductive elastic member, to contact the photosensitive drum as a contact charging member and agitate the transfer residual toner on the photosensitive drum, thereby expediting a non-patternization and, at the same time, applies a direct current voltage and an alternating current voltage and allows the photosensitive drum surface to be uniformly charged by an electrical discharge and, furthermore, achieves the xe2x80x9ccleanerlessxe2x80x9d. The peak to peak voltage and the charging efficiency of the alternating current voltage on this occasion is typically shown in FIG. 14. In FIG. 14, a point L is a stabilized discharging point and, in the case of the peak to peak voltage up to this or even more, a stabilized discharging can be executed so that a potential on the photosensitive drum comes to be almost the same converging potential as the direct current voltage applied. Because the charging potential is converged by the peak to peak voltage equivalent to the stabilized discharging point, hereinafter the peak to peak voltage, which converges the charging potential, is referred to as xe2x80x9cthe charging potential convergent voltagexe2x80x9d.
Accordingly, the charging means of a system for applying a voltage superposed with a direct current component and an alternating current component takes a constitution wherein a peak to peak voltage larger than the charging potential convergent voltage L is applied to the electrically conductive elastic member in order to stabilize a charging.
(b) On the other hand, the image forming apparatus of xe2x80x9ccleaning simultaneous with developingxe2x80x9d and xe2x80x9ccleanerlessxe2x80x9d which does not use the discharging, but uses the charging means of a direct charging system has been disclosed, for example, in U.S. Pat. No. 6,128,456. This is a system in which electrically conductive charging expediting particles are interposed in the contact portion between a contact charging member and the photosensitive drum and only a direct current voltage is applied and almost the same surface potential of the photosensitive drum as the applied direct current voltage is obtained. Because this system does not actively employ the discharging, there is no ozone generated. Further, because the system does not actively employ the discharging, an adherence of a discharging by-product on the photosensitive drum can be controlled and problems such as a smeared image under high temperatures and high humidity can be prevented.
However, in the image forming apparatus of the cleaning simultaneous with developing, which is provided with a rotary brush using the discharging operation of (a) or the charging means using the direct discharging system of (b) as shown in the above described conventionally described example, there has been the following problem.
That is, similarly to the above described (a), when an image is formed by applying the rotary brush which is the contact charging member with the alternating current voltage superposed with the direct current component, while the image forming is repeated, a toner is accumulated on the rotary brush which is the contact charging member, so that there occurs a problem of a charging defect.
This is because, assuming that the normal charged polarity of the toner as a developer is taken here, for example, as a negative polarity, the transfer residual toner at the point of having passed the transfer means is a toner having the broad distribution of a mixture of the positive polarity and the negative polarity or a majority of the positive polarity due to the influence of the positive transfer bias which is reverse to the charged polarity of the toner and therefore unable to pass through the contact charging means for the following reason and ends up adhering on the charging means.
Concretely, the transfer residual toner is, because there exists a positive toner which is a reverse toner charged with a polarity reverse to the normal polarity, drawn up to the rotary brush by the effect of the electric field working between the rotary brush and the photosensitive drum.
The transfer residual toner adhered on the rotary brush invades the discharging area which is opposite to the photosensitive drum by the rotation of the rotary brush.
In the discharging area, because the discharging has occurred, a positive charge and a negative charge are generated and the negative charge is drawn up to the side of the photosensitive drum and contributes to the charging of the surface of the photosensitive drum. Further, the positive charge generated simultaneously is drawn up to the side of the rotary brush. On this occasion, because the transfer residual toner exists on the surface of the rotary brush, the positive charge adheres on the transfer residual toner so that the toner is increasingly charged with the positive polarity.
Even when passing through the contact portion formed by the rotary brush and the photosensitive drum and the discharging area in the downstream side of the photosensitive drum rotational direction of the contact portion, the positively charged transfer residual toner is drawn up to the side of the rotary brush by the effect of the electric field working between the rotary brush and the photosensitive drum and hence remains adhered on a charging material.
By repeating the above described step, the toner which has adhered on the rotary brush is further strongly charged with the positive polarity and laminated also with a new transfer residual toner, so that the photosensitive drum becomes unable to be charged with a normal surface potential and a charging defect is caused as a result.
Further, when an alternating current voltage is made sufficiently large, because the discharging is stably generated and a discharging amount becomes large, the discharging by-product adheres on the photosensitive drum. The discharging by-product adhered on the photosensitive drum becomes low-resisting under high temperatures and high humidity and therefore an image defect such as a smeared image is apt to occur.
This phenomenon is not limited to the rotary brush alone, but even in the charging means using the contact charging member for executing a charging by a discharging operation, the above described image defect occurs for the same reason.
Further, similarly to the above described (b), in the case where the charging means of the direct charging system is used, when the number of feeding papers is increased, the transfer residual toner adhered to the charging means is accumulated and the charging means is contaminated and becomes defective, so that the defective image was often produced.
This phenomenon is due to the fact that, in the charging means of the direct charging system, the direct current voltage applied to the charging means and the charging potential on the surface of the photosensitive drum are almost the same and, for this reasons the transfer residual toner adhered on the charging means is not electrostatically discharged. Particularly, when a constitution is adapted where a velocity difference is provided between the photosensitive drum and the charging means and the transfer residual toner is actively scraped (stripped) off from the photosensitive drum, the above described image defect becomes remarkable.
It is an object of the present invention to provide an image forming apparatus, which prevents a charging defect due to contamination of a charging member.
It is anther object of the present invention to provide the image forming apparatus, which makes it easy to return a toner adhered on the charging member to an image bearing member.
It is another object of the present invention to provide the image forming apparatus, which reduces a discharging by-product produced by the charging member.
It is another object of the present invention to provide the image forming apparatus, which controls upsizing and a cost-up of the apparatus and uses the cleanerless system capable of obtaining an excellent image without generating the charging defect over a long period of time by stably executing the charging.
Still another object and characteristics of the present invention will become more apparent by referring to the accompanied drawings and reading the following detailed description.